In what may provide a potential path to processing information in a quantum computer, researchers have switched an intrinsic property of electrons from an excited state to a relaxed state on demand using a device that served ...

Harnessing quantum systems for information processing will require controlling large numbers of basic building blocks called qubits. The qubits must be isolated, and in most cases cooled such that, among other things, errors ...

A new type of "quasiparticle" theorized by Caltech's Gil Refael, a professor of theoretical physics and condensed matter theory, could help improve the efficiency of a wide range of photonic devices—technologies, such as ...

It's easy to contemplate the wave nature of light in common experience. White light passing through a prism spreads out into constituent colors; it diffracts from atmospheric moisture into a rainbow; light passing across ...

Researchers at the National Institute of Standards and Technology (NIST) have "teleported" or transferred quantum information carried in light particles over 100 kilometers (km) of optical fiber, four times farther than the ...

Symmetry permeates nature, from the radial symmetry of flowers to the left-right symmetry of the human body. As such, it provides a natural way of classifying objects by grouping those that share the same symmetry. This is ...

Physicists at UC San Diego have developed a new way to control the transport of electrical currents through high-temperature superconductors—materials discovered nearly 30 years ago that lose all resistance to electricity ...

IBM scientists today unveiled two critical advances towards the realization of a practical quantum computer. For the first time, they showed the ability to detect and measure both kinds of quantum errors simultaneously, as ...

A UNSW-led research team has encoded quantum information in silicon using simple electrical pulses for the first time, bringing the construction of affordable large-scale quantum computers one step closer to reality.

Quantum information

In quantum mechanics, quantum information is physical information that is held in the "state" of a quantum system. The most popular unit of quantum information is the qubit, a two-level quantum system. However, unlike classical digital states (which are discrete), a two-state quantum system can actually be in a superposition of the two states at any given time.

Quantum information differs from classical information in several respects, among which we note the following:

However, despite this, the amount of information that can be retrieved in a single qubit is equal to one bit. It is in the processing of information (quantum computation) that a difference occurs.

The ability to manipulate quantum information enables us to perform tasks that would be unachievable in a classical context, such as unconditionally secure transmission of information. Quantum information processing is the most general field that is concerned with quantum information. There are certain tasks which classical computers cannot perform "efficiently" (that is, in polynomial time) according to any known algorithm. However, a quantum computer can compute the answer to some of these problems in polynomial time; one well-known example of this is Shor's factoring algorithm. Other algorithms can speed up a task less dramatically - for example, Grover's search algorithm which gives a quadratic speed-up over the best possible classical algorithm.

Quantum information, and changes in quantum information, can be quantitatively measured by using an analogue of Shannon entropy. Given a statistical ensemble of quantum mechanical systems with the density matrix S, it is given by

Many of the same entropy measures in classical information theory can also be generalized to the quantum case, such as the conditional quantum entropy.